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126                          Tempering Colours.

\                        first of these is that of hardening.    Here the steel is heated as

equally as possible to a ccherry red/ and not more; and on with-
drawing from the fire it is plunged vertically into a vessel of cold
fj                              -water.    The quickness of cooling has a great effect on the hard-

I',                              ness, and this may be accelerated by moving the article about in

the water.    Cracking  or   -warping  will   also  be   prevented by
judicious motion.

The steel is now so hard that it will scratch glass. It must
next be tempered or let down to the required degree of hardness.
If the tool be again heated to cheny red, and allowed to cool
slowly it will by that means have become annealed, and will be
at its softest; but if it only be heated to one of the temperatures
,                        in the following table (Fig. 117^, Plate III.), and then cooled

rapidly, it will take a particular degree of hardness corresponding
j                        to  that  temperature, and to be obtained  at  no  other.    When

'                        let ting-down, the softest tool will be that which is cooled at the

highest temperature, and the hardest that cooled at the  lowest

The exact temperature which the tool has assumed is ascer-
tained by the colour which appears on the brightened surface, due
to a film of oxide of iron formed by contact with the air. There
is some difference of opinion as to the requisite hardness for
certain purposes, and slightly different colours are required for
, |                        different steels, bat Plate III. is suitable for average tool steel.

I                   *         Tempering a  Chisel or  Drill.To make the matter

\                        clearer we will take the case of a chisel for chipping metal.    It is

forged out of a steel bar of the section shewn at A, Fig, 122, and
is drawn out (at as low a heat as passible, to prevent burning) to
a flat point as at B,   This point is naw to be hardened and
i  ' ^                        tempered, while the rest of the chisel is to remain in its natural

condition. Whenever the tempering is accomplished by quench-
ing in water, the preliminary process of hardening must always be
performed, otherwise the tempering would have no effect. In the
case of the chisel, or any tool having a point requiring a particular
temper, tlje two processes are performed at one heat, but it must
be quite clear that hardening is not therefore dispensed with,
Keating the wtok chisel to a duny ret, the part ab only is
quenched in water, and so becomes very hard, Now rub ihe
point of the chisel with a stone to brighten it a little, aad,--asr the